The intramolecular nucleophilic cylization of haloalkenes was studied at the B3LYP/6-31+G(d) level in an Onsager continuum model for DMF(_=37.06). The reaction of (E)-2-(2-chloro-1-methylvinyl)phenol(E-1) takes a concerted in-plain σ-type S_N2 pathway with inversion at the sp^2 carbon with the activation free energy of 14.4 kcal/mol in DMF (17.1 kcal/mol in the gas phase). On the other hand, only π-addition transition structures were obtained from both Z-1 and the corresponding dichloroalkene with much higher activation free energies. These predictions were confirmed experimentally. We prepared haloalkenes 1. The E- and Z-isomers were separated by column chromatography and the geometries of the olef parts were determined by NOE experiments. When E-1 and Z-1 were treated with NaH in DMF, benzofuran was obtained in 95% yield from E-1, while Z-1 was recovered quantitatively even after heating at 110℃.